NOSTOCER, 221 
Certain genera of Nostochinee have the filaments constantly simple 
(Nostoe, Anabaena), others have the filaments frequently provided with 
lateral branches (Scytonemee, Calotrichee). Sometimes these rami- 
fications are normal, and result from the regular division of the 
trichome (Rivularia, &c.), sometimes accidental and irregular. These 
produce themselves when the trichome has broken, they grow and issue 
laterally from the sheath, when it is not rare to observe the base of the 
filaments of certain Lyngbya, or, again, when the hormogones are not able 
to escape freely, they germinate in the interior of the sheath. Indeed, 
it is very frequent that the germinations develope themselves in fila- 
ments of all ages, but in that case the young filaments are not at all 
enveloped by the general sheath of the filament that bears them. 
Nostocs should be dried as quickly as possible after they are collected, 
otherwise the trichomes escape and the sheaths are left empty.—Thwret, 
in Ann, des Sci. Nat. 6 ser., Vol. 1., pp. 372-382 (1875). 
The following is a dichotomous key to the classification as proposed by 
M. Thuret :— 
hair. : e : . 12 
rai tapering at the top into a hyaline 
1 . é < . . 
Filaments always destitute of an apical hair . . . 6.122 
Filaments in which some of the cells change 
2 | into heterocysts . . . J : : : ‘ . 38 
Filaments without heterocysts . : : a 
3 Filaments with lateral ramifications. j , . . 10 
{ Filaments without ramifications . . r . . . 4 
Filaments immersed in a gela- 
tinous mucilage of a determi- 
4 NosTocea nate shape i r . - Nostoc. 
Filaments free or immersed in an 
amorphous diffluent mucilage . . : » 5 
Filaments consisting of a coloured trichome 
5 enclosed in a hyaline sheath ‘ ‘ . 6 
Filaments without a sheath distinct from the 
trichome . ‘. F . . . . . A. B.C. 
1. Heterocysts scattered in the trichome. 
A. Spores originating in cells not adjoin- 
ing the heterocysts. . . . Anabaena, 
6B Trichomes united in small float- 
ing bundles. A z Aphanizomenon. 
B. Spores originating in cells placed on 
each side of the heterocysts . . . Spheerozyga, 
2. Heterocysts terminal (at both ends of the trichome). 
C. Spores originating in cells placed just 
below the heterocysts : . . Cylindrospermum. 
Cells disc-like. Spores very prominent. He- 
6 { terocysts placed at regular intervals . . . Nodularia. 
Cells longer than broad. Spores none . é .  Microchete. 
Filaments spirally twisted . . ‘ Spirulina. 
7 Lenesiez { Filaments not spirally twisted . ‘ . . 8 
